There are three main classes of laser-material interaction: photocoagulation, photoablation, and photodisruption. Photocoagulation employs continuous wave laser light applied to absorbing material targets, with effects mediated by primary and secondary effects of thermal damage. This technique is most widely used in the eye to treat retinal diseases, such as diabetic retinopathy and macular degeneration. In photoablation, highly absorbing ultraviolet wavelengths are used to vaporize superficial materials, primarily for surface etching and refractive surgical applications in the cornea.
Photodisruption begins with laser induced optical breakdown (LIOB), when a laser pulse generates a high intensity electric field, leading to the formation of a mixture of free electrons and ions that constitutes the plasma state. The hot plasma expands displacing surrounding material. As the plasma expansion slows, the displacement front propagates through the material as a shock wave. The shock wave loses energy and velocity as it propagates, relaxing to an ordinary acoustic wave. The adiabatically expanding plasma quickly recombines and cools down, eventually forming a cavitation bubble. The constituents of the cavitation bubble depend on the make-up of the target material. For organic substrates, the cavitation bubble consists mainly of CO2, N2 and H2O.
Using a photodisruptive laser and a delivery system, localized photodisruptions can be placed at or below the surface of a material to produce high-precision material processing. In one example of such material processing, internal surfaces can be created within the material by placing multiple pulses along a predetermined path. In special cases, these surfaces can be represented as planes placed in any orientation to create horizontal, vertical or oblique effects.
In using photodisruptive lasers, variable outcomes can result from the disposition of gas, debris, and other photodisruptive by-products. In some materials, photodisruption results in formation of gas and water vapor. The behavior and effects of these and other by-products depend on the properties of the material surrounding them, as well as on the influence of additional laser pulses placed subsequently in the near vicinity. Generally, a gas bubble expands in size into the area of least resistance. With expansion, the gas cools and constituent gases, such as water vapor, can return to a liquid state. The presence of gas, liquid, debris and other by-products created during photodisruption in the region where additional laser pulses are being placed can be a cause of variable or undesired outcomes. The current invention represents an improvement over previous techniques utilizing laser photodisruption by offering elimination or mitigation strategies against these potential influences.
A specific application of the invention is in the use of a photodisruptive laser for the creation of a corneal layer in ophthalmic surgical procedures to correct vision errors. Vision impairment can occur for many reasons, and be the result of many causes. One common cause for vision impairment results from a defective condition of the eye which occurs when the refractive characteristics of the cornea do not cause parallel rays of light to focus on the retina. When the eye is at rest, and the rays of light focus in front of the retina, the condition is known as myopia (i.e. near-sightedness). On the other hand, when the rays of light focus behind the retina, the condition is known as hypermetropia or hyperopia (i.e. farsightedness). Both myopic and hyperopic conditions result in varying degrees of vision impairment. In most cases the conditions are correctable.
Eyeglasses or contact lenses are commonly used to correct myopic or hyperopic conditions. For various reasons, however, many persons who suffer with these conditions prefer not to wear eyeglasses or contact lenses. Alternative ways to correct these conditions include known surgical procedures for reshaping the cornea in various ways that are effective in changing its refractive characteristics. For example, in U.S. Pat. Nos. 4,665,913 and 4,669,466 to L'Esperance, a laser system is described which photoablates corneal tissue from the anterior surface of the eye. Another procedure is described in U.S. Pat. No. 4,988,348 to Bille, whereby corneal tissue is first removed to correct vision, and then the newly created surface is smoothed.
Rather than remove and reshape portions of the anterior portion of the eye to correct refractive defects, other procedures have been developed using a technique called intrastromal photodisruption for removing internal stromal tissue. Examples of such procedures and laser systems are described in U.S. Pat. No. 4,907,586 to Bille et al and U.S. Pat. No. 5,993,438 to Juhasz et al. Another example of a procedure for removing stromal tissue is the procedure described in U.S. Pat. No. 6,110,166 to Juhasz. In this procedure, an anterior corneal layer can be defined by using a laser to create a series of overlapping photodisrupted areas. The surgeon then separates the corneal layer by lifting it, to gain access to the underlying corneal tissue, the shape of which is changed with a photoablative laser, such as an excimer laser. The corneal layer is then repositioned on the cornea.
In prior practice, surgeons would create a corneal layer by focusing the laser beam at a starting point at or near the center of the to-be-formed corneal layer. The laser beam begins photodisrupting areas of tissue at the starting point, and is moved along a predetermined path, typically in a spiral pattern, from the center of the corneal layer to a predetermined circumference of the corneal layer. Finally, the laser beam is directed around the predetermined circumference to form a peripheral cut from the corneal layer to the outer surface of the cornea.
It has been observed in some cases that moving the laser beam along a predetermined path or pattern creates a temporary cloudy appearance, which is believed to result from gas and/or debris created during the photodisruption process that spreads inside the tissue because there are no outlets for the gas and debris. This condition is temporary; the gas and/or debris are eventually absorbed in surrounding tissue after a few minutes. Although this condition has no signficant side effects, spread of gas can influence the effectiveness of further laser pulses placed in the predetermined path in creating a high quality internal surface. Both the cloudy appearance and the less effective effects of ensuing laser pulses are considered undesirable.
It has also been observed that, in some cases and situations, moving the laser beam along a predetermined pattern creates fluid that can spread in the tissue and influence the effectiveness of further laser pulses placed in the predetermined path in creating a high quality internal surface. This fluid may result in surface irregularities that reduce the smoothness of the newly-created internal surface.
Thus, there is a perceived need for a predetermined path or pattern that does not cause gas and debris to spread in the tissue, or that does not lead to the above described creation or spread of fluid, both of which alter the character and effectiveness of further pulses placed along the predetermined path. As an alternative, even if these gas and/or fluid effects cannot be eliminated, the impact of these negative secondary effects also can be mitigated by choosing specific predetermined paths. Additionally, a need exists for a method and system to implement these desirable photodisruption patterns and predetermined paths.
A specific example of a desirable pattern or predetermined path involves creation of a secondary pattern or predetermined path connected or adjacent to the primary pattern or predetermined path. This reservoir can control the effects of by-products and/or gas from any type of pattern cut. Additionally, a need exists for a method and system to implement the reservoir. In conjunction with such a reservoir, or as an alternative approach, specific laser pulse placements and characteristics in the primary or secondary pattern or predetermined path can be chosen to control the effects of by-products and/or gas from any type of pattern cut.